Warnings cleanup

On windows Wall contains all possible warnings (most of which is either
generated by windows headers or too pedantic - e.g. padding added by compiler)

and remove comment regarding clang-format (only 3.8 and 3.9 is supported)

to avoid -Wformat-security

common: update docker images

common: set codecov threshold to 0.2

common: fix cmake variable name in readme

tests: better message while skipping tests

doc: fix reference to v snippet

Add PMEMOBJ++Config.cmake and build standalone examples

and some cleanup

doc: add link to v template

common: deal with dot files

examples: add standalone CMakeLists

common: auto doc update

common: fix finding libpmemobj

Code for finding libpmemobj was not properly removed.

CMake files refactoring

common: use explicit conversions

to avoid sign-conversion warnings. All values used in indexing and persistent_ptr
arithmetic operations are explicitly casted to uint64_t.

Based on well-defined unsinged overflow arithmetic and standard guarantees about integer
conversions:

[conv.integral]
If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source
integer (modulo 2
n where n is the number of bits used to represent the unsigned type).

Do not require libpmemobj when tests and examples are not built.

common: detect pmemvlt support in libpmemobj

Current required version of libpmemobj is 1.4 which does not have
pmemvlt support. This patch turns off and building tests and examples
using it if pmemvlt is not provided by libpmemobj.